DESCRIPTION: (Scanned from the applicant's abstract) Spermatogenesis relies on the establishment and maintenance of a stem cell population within the testis. Although spermatogonial stem cells are essential for reproduction, little is known about their regulation. Environmental cues from nearby cells are thought to be crucial for stem cell maintenance, but identifying them is extremely challenging in mammalian systems. Drosophila spermatogenesis provides an excellent model system for studying stem cell regulation, since spermatogonial stem cells can be identified, and genetics can be used to systematically identify regulatory molecules. In this proposal the role of the highly conserved Jak-STAT signal transduction pathway in stem cell maintenance is examined. The Jak kinase homologue Hopscotch (Hop) is required for stem cell maintenance, and overactivation of the Jak-Stat signaling pathway leads to ectopic cells with stem cell character. Also, the ligand activating Jak-STAT is present in a small group of somatic cells, called the hub, to which the stem cells are anchored. This leads to the hypothesis that the hub comprises a stem cell niche, or specialized local environment, that instructs nearby cells to retain a stem cell fate by activating the Jak-STAT pathway within these cells. To test this hypothesis, the precise role of Jak-STAT signaling in the niche will be examined. In Aim 1, marked loss-of-function clones will be generated to determine if stem cells directly require Jak-STAT signaling. In Aim 2, both loss-of-function and ectopic expression of Jak-Stat signaling molecules will test whether Jak-STAT signaling instructs stem cell fate or, alternatively, is required to permit stem cell viability. In Aim 3, the role of Jak-STAT signaling in limiting stem cell numbers in the niche will be studied. Finally, since it is likely that other factors act either in concert with Jak-STAT, or subsequent to it in maintaining the stem cell niche, we employ genetic approaches to systematically identify these factors in Aim 4. Since Drosophila and mammalian spermatogenesis are conserved, the regulatory mechanisms uncovered in this proposal will likely add to the understanding of the regulation of stem cells residing in more complex and less defined environments, such as mammalian spermatogonial stem cells, which are essential for human fertility.